There are a variety of DC-to-DC power converters available for transforming an input DC voltage of one magnitude to an output DC voltage of another magnitude. In general, switching power converters function by operating an active switch to alternately connect and disconnect a source voltage to a load. In order to deliver continuous power to the load, the power converter must store energy for use during periods in which the switch is open and the voltage source is disconnected. The amount of energy that must be delivered to the load by the energy storage device, and therefore the amount of energy that must be stored in the energy storage device, is related to the switching frequency. Consequently, by increasing the operating frequency of the power converter, the energy storage capacity is reduced. Similarly, the size and weight of the power converter may be reduced by increasing the switching frequency.
There are at least two primary types of power converter topologies. In a "flyback" converter, a voltage source is connected through a switch in series with the input winding of a transformer. By alternately opening and closing the switch, a pulse is produced in the secondary winding of the transformer, which is connected through a diode to an output capacitor. Because the switching rate and the rates of change in the current in both the primary and secondary windings are very high, electromagnetic and radio frequency interferences are produced in the circuit. Filters must be used to attenuate this interference, increasing the complexity and cost of the system, and diminishing its efficiency.
In a "forward" converter, an inductor is typically added after the secondary winding of the transformer, to reduce the absolute current magnitude in the secondary circuit. A second diode is also included in the secondary circuit to close the circuit between the output capacitor and the inductor when the input switch is opened. As with the flyback converter, electromagnetic and radio frequency interferences are produced in the forward converter, which requires filters to attenuate. For both the flyback and forward converters, simultaneous occurrence of a voltage across and a current through the switch occurs, resulting in a dissipation of energy. The net result is an overall reduction in efficiency and reliability due to the higher component operating temperature.
More recently, many of the limitations found in the forward and flyback converters have been addressed by improvements found in the "resonant" power converter. In the resonant power converter, the converter is tuned or components are added to the power converter to establish an effective LC circuit that defines the time scale for the rise and fall of energy through the windings of the transformer. By taking advantage of this energy rise and fall, or resonance, one or more switches in the switching power converter can be switched open and closed at zero current through the switch, zero voltage across the switch, or both. By switching at either zero-voltage or zero-current switching losses that result from the dissipation of energy during the simultaneous occurrence of a voltage across and current through the switch are reduced. At the same time, much of the noise produced by the switches can be eliminated.
Although resonant power converters generally have less switching loss than previous typologies, they still suffer to a larger degree to electromagnetic and radio interference produced in the circuit. Consequently, even resonant designs typically include input filters to attenuate these interferences.
In addition, as with previous power converters, the performance of resonant power converters generally suffers due to influences such as drift, input line or load impedance variations, or other external factors. Such influences, particularly including changes in the input line or load impedance, typically cause corresponding changes in the resonant frequency of the power converter. Because of this frequency variation, the power converter filters must be designed in a manner to filter noise across a wider frequency range. This design constraint adds expense and complexity to the design. Further, many applications simply cannot tolerate such frequency variations. For example, when the frequency of a power converter associated with a cathode ray tube (CRT) is asynchronous with the raster scan rate of the CRT, the resulting noise may produce visual effects, such as hum bars, on the CRT screen. In addition, this frequency variation may cause an otherwise resonant power converter to switch at times when there is voltage across, or current through the switch, causing additional noise and inefficiencies.
Although some power converter designs are intended to operate resonant at a fixed frequency, they are only able to maintain resonance over a fairly narrow range of input line or load impedance variations, which depending on the application, could become a performance limitation.